The invention relates to a method of fabricating an electrically conductive mechanical interconnection element, and to a method of making a mechanical interconnection.
The invention applies in particular to the field of electronics.
In electronics, it is constantly necessary to make electrically conductive mechanical interconnections.
This applies in particular to connecting the pins of a microelectronic chip to a track or an area of metal plating on a printed circuit for making an electronics card.
Typically, such electrical and mechanical connections are made by soldering or by thermo-compression. Those conventional techniques present numerous drawbacks. Firstly they require high temperatures to be used, which can damage fragile electronic components. Secondly they are not easily taken apart. Unfortunately, it is often important to be able to remove and replace a chip of an electronics card, for test, repair, and/or replacement purposes.
Interconnection problems are particularly severe with electronic components for three-dimensional integration performed by stacking and interconnecting planar devices such as chips or miniature printed circuits. Once such an assembly has been made, it is generally not possible to take it apart without destroying the planar elements making it up.
Document EP 1 583 146 discloses a nanostructured interconnection element constituted by a metal surface from which there project conductive wires of sub-micrometric diameter (“nanowires”). A mechanical and electrical interconnection is made by pressing said surfaces one against the other. This causes the nanowires to tangle together, thereby creating a bond between the surfaces. The interconnection obtained in that way can be undone merely by applying sufficient traction force, and then it can be re-established by a new compression step.
Document EP 1 583 146 does not describe in detail any method for enabling such an interconnection element to be fabricated, being content merely to cite prior document U.S. Pat. No. 6,185,961.
That document describes a method of fabricating a plurality of metallic nanowires projecting from a surface, the method comprising: fabricating a porous matrix out of glass; depositing a layer of a noble metal on one face of said matrix by vaporization or spraying; fabricating nanowires by electrochemical deposition in the pores of the matrix; and dissolving the matrix to release the resulting nanostructured metallic element. That method is very complex and expensive, particularly concerning the steps of fabricating and metal-plating the matrix. Furthermore, the need for a step of depositing an electronically-conductive thin film by spraying or by vaporization limits the range of materials that can be used. For example, it is not possible to make a nanostructure entirely out of copper.
Other methods of fabricating structures constituted by a plurality of conductive nanowires projecting from a surface are known in the prior art. Nevertheless, those structures are found to be unsuitable for making mechanical interconnections by tangling nanowires together.
For example, the article by S. Fiedler et al. “Evaluation of metallic nano-lawn structures for application in microelectronic packaging” describes a method of fabricating such a nanostructure by an electrolytic method generally similar to that of document U.S. Pat. No. 6,185,961, except in that the glass matrix is replaced by a polymer membrane in which pores are formed by bombarding it with particles. The drawback of that method is that the pores obtained in that way (and thus the nanowires that are deposited therein) are not rectilinear and their disposition is random. Experience shows that a satisfactory interconnection cannot be achieved in that way. At best, the method enables nanostructures to be created on metal-plated areas of microelectronic chips or printed circuits, thereby making them easier to connect by soldering.
Document WO 2006/123049 discloses yet another method of fabricating a nanostructure constituted by a plurality of metallic nanowires projecting from a surface of a substrate. That method likewise provides for forming nanowires by electrochemical deposition in the pores of a sacrificial matrix. The originality of the method lies in using a matrix constituted by a membrane made of a porous ceramic material, and more particularly of alumina. The method of document WO 2006/123049 is advantageous since the pores of alumina membranes are substantially rectilinear and they are arranged in a regular array. In addition, there is no need to proceed with a prior step of metal-plating said membrane by vaporization or spraying.
Nevertheless, the present inventors have observed that the structures obtained in that way do not enable mechanical interconnections to be obtained by the nanowires interpenetrating and tangling together, even though they have performed a plurality of tests with various different commercially-available porous alumina membranes. Document WO 2006/123049 does not describe such an application.